1. Field of Invention
The invention is generally related to apparatus for measuring the intensity of a light beam and is specifically directed to a circuit for measuring the intensity of light by correlating light intensity to a time signal.
2. Description of the Prior Art
There are numerous applications where the intensity of a light beam passing through a medium is indicative of conditions existing in the medium. For example, Abbott Laboratories, the assignee of the subject application, has developed a number of chromatographic immunoassay analyzers for the determination of certain conditions existing in organic samples. The assay typically utilizes a single disposable housing with a strip of laminated nitrocellulose or the like attached to a glass fiber pad. The assay is exposed to a reference light source which is passed through the medium, The emitted light is collected at a sampler for measuring its intensity. The intensity of the light is indicative of the conditions of the sample. This type of measurement has been particularly useful in screening and providing a qualitative human chorionic test for prompt detection of pregnancy and evaluation of problem pregnancies.
Light intensity detection circuits for use with these types of analysis schemes are relatively well known. Typically the light intensity is detected by using a photosensitive element which is capable of generating a voltage signal which is then amplified and converted to a digital signal by an analog-to-digital (A/D) converter. The A/D converter generates a variable level signal for introduction into a microprocessor, wherein the signal level is compared to a reference signal in order to correlate the collected signal with a predetermined reference to "read" the sample.
Examples of A/D converters are shown and described in U.S. Pat. No. 4,779,074, entitled: Low Level Voltage Pulse Converter, issued to R. E. Whitford, :et al. on Oct. 18, 1988, and U.S. Pat. No. 4,178,585, entitled: Analog to Digital Converter, issued to K. Takagi, et al. on Dec. 11, 1979.
The known devices for utilizing light intensity sampling in order to determine conditions of a sample universally use A/D converters along the lines described in the above patents to determine the intensity of the light by producing a converted digital signal of varying levels, corresponding to measured light intensity. For example, European Patent Application No. 0479394A3 by R. Phillipps, et al., filed on Aug. 7, 1987, discloses a method and apparatus for determination of analytes. A light source is directed toward a sample and a detector collects the emitted light. The intensity level of emitted light is utilized to determine the condition of the sample. As shown in that application, the emitted light is converted into an analog signal which is then amplified and converted into a digital signal through an A/D converter, with the digital output of the converter being introduced into the microprocessor. U.S. Pat. No. 4,766,083 entitled: Method for the Photometric Determination of Biological Agglutination, issued to M. Yoshinobu, et al. on Aug. 23, 1988, discloses a photometric method and apparatus for measuring agglutination in a biological agglutination reaction system test sample using a laser beam source and a photodetector for detecting light scattered by the test sample. The light collected by the photosensitive element is converted from an analog signal to a digital signal before processing.
While these devices provide accurate sampling techniques, and the analog to digital conversion scheme is suitable for accomplishing the desired result, there remain disadvantages to the use of A/D converters in light intensity analysis systems. Primarily, the cost associated with high resolution A/D chips has prohibited the widespread adoption of light spectrum analysis systems. A typical analog to digital converter network having a 16 bit converter capability can cost several hundred dollars. Where 24 bit resolution is required, the cost can increase by dramatic proportions. Thus, one of the most expensive, prohibitive components in the system is the converter network. Therefore, if the cost of the signal conversion can be greatly reduced, the applicability of light spectrum analysis equipment can be greatly enhanced at reduced cost, providing better testing capability for broader based sampling systems.
At present, there remains a need to develop a system permitting the application of light intensity detection systems utilizing conversion equipment in an efficient yet accurate effective manner.